Significant cost reductions in photovoltaic panels have spurred the installation of solar energy systems for supplying electricity. Solar panels produce DC power and are generally followed by a DC to AC converter, also known as an inverter, to produce a power output at the electric utility grid frequency of 50 Hz or 60 Hz, depending on the country. The most common solar installation feeds the AC power it produces back to the grid in return for some economic benefit. Relatively few solar installations are designed to operate totally off-grid, which requires a very large and expensive battery, to bridge periods of low insolation.
U.S. Pat. No. 8,937,822, filed May 8, 2011 by the present inventor and titled “Solar Energy Conversion and Utilization System,” incorporated herein by reference in its entirety, discloses a beneficial system that does not aim to be totally off-grid but rather attempts to minimize grid power use and maximize use of self-produced solar energy. An adaptive power source selection unit is used to transfer as many loads as possible, at any given time, to solar while leaving the remainder on the grid. Such a system requires a much smaller battery than a totally off-grid system, yet still provides substantial back-up power in the event of grid outages. As the grid becomes saturated with fed-back solar power, the self-consumption system of the '822 patent is becoming of greater interest.
The '822 patent also describes a novel type of transformerless DC to AC inverter that results in size, cost and weight reductions. As used herein, the term “transformerless” refers to the absence of the very large transformers that are needed to handle power at 60 Hz. The term does not imply the total absence of wound components, which may still be used at lower power at high frequency, where their size and cost is considerably lower than those used for high power at low frequency. Transformerless inverters of other types are also possible. In particular, transformerless inverters may be designed and used to feed power back to the electric utility grid.
When power is fed back to the grid from a solar inverter, it is connected to a service drop transformer such as a pole-mounted transformer that steps the inverter voltage up to a high transmission voltage. Such utility transformers have very low winding resistance and would substantially prevent any small DC component from existing on the grid wiring. Moreover, grid-tie inverters are of a current source type, not a voltage source type, as the voltage they produce is determined by the grid. A current source inverter controls the current to produce negative and positive half sine waves of current of a determined peak value, and as such they inherently prevent any DC current build-up that would result from a DC bias on the inverter output.
However, inverters for self-consumption have to produce a sine wave of defined voltage. If voltage is controlled rather than current, it must be controlled in such a way that no significant DC bias is produced by mismatch between the positive and negative half cycles. A DC bias is undesirable for feeding appliances with transformers or motors that have a low DC winding resistance, which would result in a significant DC current bias occurring. For example, one way in which such a bias can result even in a well-adjusted inverter is if one of the loads contains a half-wave rectifier, as old vacuum tube TV sets often did. This results in one half cycle being more heavily loaded than the other, resulting in a voltage drop on one half cycle and not on the other. This is perceived by other appliances as a DC offset. In effect, other appliances assume some of the unbalance current of the half-wave rectifier, which is undesirable. Therefore there is a need in the art for circuits and methods to control the DC bias output from a transformerless voltage-source inverter.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Approaches described in the Background section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.